From my experience, the battle was simple, but hard to pull off. You see, I'm using (in my opinion, without sounding too selfish) best and well mixed Pokémon with each of their roles set in.
These are:
Slowking (Last Plot Kill) ; Krookadile (Anger Point Destroyer) ; Roserade (Toxic Spiker) ; Magnezone (Analytic Tank) ; Froslass (Destiny Bonder), and finally, my signature Pokémon:
Blaziken (Speed Bulking Blazekicker)
While I don't know much about your Pokémon moveset, I will, however, give you one strategy that I used on most of my Gym Battles whenever I cannot